The actions of adenosine on several adenylate cyclase systems have been investigated. Adenosine inhibits hepatic adenylate cyclase at a site which, based on analog studies, is markedly different from the site through which the nucleoside activates the Leydig cell cyclase. The adenylate cyclase in platelets contains two sites for adenosine action which appear identical to the sites identified in liver and Leydig cells. Thus, adenylate cylase systems may contain one or the other, or both types of adenosine receptor. These studies provide a basis for testing whether any of the many biologic effects of adenosine are mediated by adenosine acting through the adenylate cyclase system. BIBLIOGRAPHIC REFERENCES: Londos, C. and Rodbell, M.: Adenylate Cyclase: Actions and Interactions of Regulatory ligands. In: Drug Action at the Molecular Level (Ed.G.C.K. Roberts) London, Macmillan Press; 235-247 (1977). Rodbell, M. and Londos, C.: Regulation of Hepatic Adenylate Cyclase by Glucagon, GTP, Divalent Cations, and Adenosine; Metabolism 25 (Suppl. 1) 1347-1349 (1976).